Training for Non-Radiologists

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Nov 15, 2013 (4 years and 11 months ago)

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Training for Non
-
Radiologists

















To Perform


Radiographic Studies


SUNY Upstate Radiation Safety Office
Contents




Unit 1
-

Principles of Equipment Function



Unit 2
-

The Biological Effects and Significance of X
-
Ray Exposure



Unit 3
-

Radiation Exposure Standards



Unit 4
-

Radiation Protection


1
-
1

UNIT

1: PRINCIPLES OF EQUIPMENT FUNCTION


1.1

Radiological Terminology


ROENTGEN RAY. This term is synonymous with X
-
ray.


ROENTGENOLOGY. This study and use of radiation; synonym for radiology.


ROENTGENQGRAM. Synonym for radiograph which is the record of an image
produced by the passage of X
-
rays through an object.


ATOM. The smallest particle of a substance that can

exist and still retain the properties of
that substance. It is composed of the

following:


1)

Nucleus. The positively charged center of an atom containing most of the mass
in the form of protons and neutrons.


2)

Proton. A positively charge particle having mass or weight.


3)

Neutron. A neutrally charged particle also having mass or

weight.


4)

Electron. A negatively charge particle having very little mass orbiting the
nucleus at various distances.


MOLECULE. A group of two or more identical or different atoms.


ION. A charge particle (either positive or negative) resulting from the
ionization of atoms
or molecules.


IONIZATION. The process of adding electrons to, or knocking electrons from, atoms or
molecules, thereby creating ions.


1.2

Definition and
Types

of Radiation



Radiation may be defined as the process in which energy in th
e form of rays of
light, heat, or X
-
rays, is sent out from atoms and molecules as they undergo internal
change. Basically, there are two types of radiation, particulate and electromagnetic.
Particulate radiations result from the splitting of atoms, such
as alpha and beta particles
given off by radium. Electromagnetic radiations are pure energy without mass or weight.

X
-
rays, light, heat, radar, and radio waves are examples of electromagnetic radiations.
Light or radiant energy travels as a wave motion a
nd, therefore, wavelength is its one
measurable characteristic. All forms of electromagnetic radiation are grouped according
to their wavelengths in what is called the electromagnetic spectrum. In medical and
dental radiography, X
-
rays have a wavelength
of about one billionth of an inch. X
-
rays
also act as if they consist of small, separated bundles of energy, and they can be
understood if a beam of X
-
rays is considered as a shower of particles. This dual nature of
X
-
rays, wave
-
like and particle
-
like, i
s inseparable.




1
-
2

1.3

Properties of X
-
rays



X
-
rays are invisible waves of electromagnetic energy that travel at the speed of
light (about 186,000 miles per second or 3x10
8

meters per second) and in straight lines in
all directions. X
-
rays cannot be seen or

felt, but they have properties, which make them
valuable in diagnosis and treatment. X
-
rays, if not properly controlled can be harmful in
stimulating and destroying living tissue. X
-
rays have the ability to penetrate opaque
material, to affect the sensiti
ve emulsion of photographic and radiographic film the same
as light does, and to produce fluorescence in certain chemical compounds.


1.4

Production of X
-
rays


1.4.1

Electrical Terms


ELECTRIC CURRENT. A flow of electrons from one point to another.


ELECTR
ON. A negatively charged particle.


DIRECT CURRENT. A current of electricity which flows in one direction.


ALTERNATING CURRENT. A current of electricity which flows first in one direction
and then reverses and flows in the opposite direction.


CYCLE. One
forward and one reverse flow of an alternating current.


AMPERE. The unit of current flowing through a circuit.


MILLEAMPERE (mA). One
-
thousandth (1/1,000) of an ampere.


MILLIAMMETER. Au instrument that measures milliamperage.


MILLIAMPERE
-
SECONDS (mAs).
The number of milliamperes of electricity flowing
around a circuit in one second.


VOLT. A unit of measurement of electrical pressure which forces a current through a
circuit. A kilovolt equals 1,000 volts.


VOLTMETER. An instrument that measures voltage.
In an X
-
ray unit, it measures the
voltage of the current before that voltage is stepped up by the transformer.


WATT. A unit of measurement of electrical power. The voltage times the amperage
equals the wattage. One volt times 1 ampere equals 1 watt.


OHM.

A unit of measurement of electrical resistance. It requires 1 volt to force 1 ampere
through 1 ohm of resistance.



1
-
3

1.42

X
-
ray Producing Equipment



An electric current is the flow of negatively charged particles called electrons. This
flow normally occurs

along wire conductors. If the voltage (pressure) is high enough,
electrons will cross a gap between conductors. When electrons, which are rapidly moving
across such a gap, are suddenly stopped by hitting a metal target, X
-
rays are produced.
The following
comparison may give a clearer understanding of X
-
ray

production. If a
handful of marbles is thrown at a drumhead, the marbles will lose their velocity when
striking the drum but will cause the drumhead to vibrate. This vibration sends sound
waves into the
air. The marbles represent the electric current. The drumhead corresponds
to the target of the electronic tube in which X
-
rays can be generated. The resulting sound
waves in air correspond to the X
-
rays produced in the tube.


1.4.3

X
-
ray Beam Generation



X
-
rays are similar to visible light rays in that they radiate from the source in all
directions unless they are stopped by an absorber. For this reason, the X
-
ray tube is
inclosed in a metal housing that stops most of the radiation and permits a beam to le
ave
the tube only through a “window” in the tube housing. The beam of useful radiation is
composed of rays of different wavelengths and penetrating power.



In conventional X
-
ray generators a metal anode is bombarded by high kinetic
energy electrons, as s
hown in Fig. 1
-
1. When electrons in a conventional generator strike
the anode, roughly 99% of them experience nothing spectacular; they undergo sequential
glancing collisions with particles of matter, lose their energy a little at a time and merely
increas
e the average kinetic energy of the particles in the target; the target gets hot.
Perhaps one percent of the incident electrons contribute a photon to the X
-
ray spectrum.





Figure 1
-
1: Schematic Diagram of an X
-
ray Generator. The heated filament boils o
ff

electrons, which then accelerate toward the positively charged Cu anode. The photons
are absorbed by shielding and collimators (not sh
own), except those headed along
the

main beam axis. (After Piccard and Carter, 1989.)




1
-
4



Two mechanisms produce X
-
ray
photons: first, the sudden deceleration of the
electrons produces photons with a wide distribution of energies, ranging from very tiny
up to the kinetic

energy of the incident electrons. These photons (known as
"bremsstrahlung," German for "braking radiati
on") have a continuous spectrum with a
broad peak of intensity for photons with roughly half the incident electron energy, and are
more numerous in directions perpendicular to the electrons' acceleration vector.



Roughly as many X
-
rays are generated by t
he second mechanism, which begins
when an incoming

electron "chips off" an inner electron from an atom of the anode.
Following such an ionization event,

an outer electron drops down to fill the vacancy,
emitting a photon of energy equal to the difference

b
etween the binding energies of the
old and new states of this (orbital) electron. Photons of this sort

have energies that are, of
course, characteristic of the anode material, and are emitted from the atom

with equal
probability in all directions. We thus
expect the spectrum of a conventional X
-
ray

generator to show a combination of continuous and line features.



1.4.4

Types of Radiation Produced by a Radiographic Unit



Primary Radiation. Primary radiation includes all radiation that comes directly
from
the anode. Except for the useful beam, it is absorbed by the tube housing.



Stray or Leakage Radiation. Stray radiation is any radiation that does not serve a
useful purpose. This category includes radiation coming from the tube head through a
crack or j
oint in the tube housing.



Secondary Radiation. Secondary radiation is that radiation emitted by any
substance through which X
-
rays are directed or by any irradiated material.



Scattered Radiation. Scattered radiation is that radiation that has been devi
ated in
direction during its passage through a substance. It may also have been altered by an
increase or decrease in wavelength.


1.5

Penetrating Power of X
-
rays



X
-
rays vary considerably in the ability to penetrate matter. Those of relatively short
wave
lengths have greater penetrating power than those of longer wavelengths. The
wavelength is determined by the positive voltage applied to the anode of the tube to
attract the negative electrons emitted by the filament. The higher the voltage applied, the
sh
orter the wavelength will be. Desired adjustments in penetrating power can be made by
changing the anode voltage.



The degree to which X
-
rays penetrate a substance also depends upon the nature of
the substance or density. X
rays will penetrate the soft t
issue of the lips and cheeks very
readily, but the bone and hard structure of the teeth are much more resistant. The greater
the density of an object, the more radiation, it will absorb. Lead is one of the more dense

1
-
5

substances and will absorb most radiati
on, a property which makes it useful in protection
from radiation.


1.6

Production of a Radiographic Picture


1.6.1


General.



Because of the difference in the degree to which X
-
rays will penetrate different
tissues and substances, the amount of radiatio
n reaching any portions of the film will
determine the degree to which that portion of the emulsion of the film is affected.
Differences in these effects in different portions of the film produce a photographic
record of whatever lies between the film and
the source of the X
-
ray.


1.6.2


Radiolucency.



An object through which radiation passes freely is called radiolucent. The area on
the film corresponding to a radiolucent area receives more radiation than the surrounding
area; therefore, after processing
, the area corresponding to a radiolucent object is
considerably darker. All soft tissue is radiolucent.


1.6.3


Radiopacity.



The direct opposite of radiolucent is radiopaque. A radiopaque object or area is one
which tends to absorb the radiation; there
fore, the film is less exposed to radiation. These
areas on a radiographic film appear light in contrast to the radiolucent areas. Bony
structures are radiopaque areas. Due to the various densities in tooth structure, the
radiographic film is exposed to va
rying degrees of radiation. The image is made of
varying shades of light and dark areas corresponding to varying degrees of radiolucency
and radiopacity.


1.6.4


Distance.



As the distance increases, the radiation intensity at the object decreases. This
is a
particularly important factor when cones of different lengths are used.


1.7

Dosage Measurements



The methods of measuring the energy, or quantity, of ionizing radiation involve the
physical characteristics of the radiation. The photographic methods
determine the
quantity by the degree of darkening of a sensitive photographic emulsion. The personnel
monitoring film badge is an example of that method. The fluorescent, chemical, and
thermal properties are also used to determine the quantity of radiation
, but the most
important method of measurement utilizes the ability of radiation to ionize gases. The
ions can be counted because of their electrical charge and resulting electrical current. The
instrument used to determine the amount of ionization is call
ed the standard free
-
air

1
-
6

ionization chamber.



The radiation energy necessary to expose X
-
ray film is expressed as milliampere
-
seconds of electric current used to produce the X
-
rays. Milliampere
-
seconds can be
converted to appropriate X
-
ray dosage at give
n distances from the X
-
ray tube target if the
total filter equivalent, milliamperage, and voltage of the current, and the exposure time
are known. Dosages at other distances vary inversely with the square of the distance. This
means that doubling the dista
nce from the X
-
ray tube target reduces the amount of X
-
ray
reaching a given area of tissue to one
-
fourth of its value. The mAs is computed by
multiplying the mA by the total number of seconds exposed.



1.8

Fluorescence and its Use

in Radiography



The abi
lity of X
-
rays to cause certain substances to fluoresce (glow) is the principle
employed in fluoroscopy. Fluorescence is also used to intensify the exposure of X
-
ray
film by means of visible light.



An intensifying screen is a device which augments the p
hotographic effects of X
-
rays, decreasing the amount of radiation required. It consists of a thin, pliable material
coated with a substance which fluoresces when exposed to X
-
rays. When used, the light
emitted by the fluorescing surface intensifies the eff
ect on the emulsion of the X
-
ray film
so that less X
-
ray exposure is needed.



A fluoroscope is a device with a screen which fluoresces when exposed to X
-
rays,
forming an image of the structure through which the rays pass. Use of a fluoroscope
provides an
immediate and direct means of examining tissues.



Radiologists recognized many years ago that the poor visibility of image detail was
related to the dim image presented by a conventional fluoroscope. These persons
emphasized the need for brighter fluorosc
opic images and encouraged the development of
the image intensifier. Image intensifiers increase the brightness of the fluoroscopic image
and permit the observer to use photopic vision. Hence, dark adaptation is not required for
fluoroscopy with image inte
nsification.



2
-
1


UNIT 2: THE BIOLOGICAL EFFECTS AND

SIGNIFICANCE OF X
-
RAY EXPOSURE


2.1

Introduction



The physical basis for the biological consequences of ionizing radiation exposure
is the transfer of energy to the biological organism. The energy transfe
rred to matter from
ionizing radiation produces ionizations (electron missing from atom) of atoms and
molecules. The deposited energy also produces excitations (electron vacancy in a shell) of
atoms and molecules in the absorbing material. These ionization
s and excitations can lead
to permanent changes to the tissue, which may result in demonstrable biological injury.



The specific mechanisms involved in radiobiological injury are not completely
understood; however, nucleic acids are probably involved in t
he more serious effects.
Small modifications of DNA structure can have widespread consequences for the cell,
because the structure of a DNA molecule constitutes the cell’s operational “program”. In
addition, since DNA is replicated during mitosis, any poin
t mutation may be perpetuated
in the cell’s progeny. For example, a particular alteration could result in the synthesis of
enzymes which differ from the normal in time of production, spatial distribution, or
configuration. Depending upon the relative impor
tance of the particular enzyme changes,
their activity, and the frequency of their production, the effects upon the cell as a whole
can range from insignificant metabolic alterations to severe interruption of normal
function.



The severity of radiobiologi
cal injury is also clearly dependent upon the location of
the initial radiation interaction. For example, small alterations in the protein synthetic
mechanism occurring in the cytoplasm of the cell might cause localized damage but
would be unlikely to gene
rate large scale changes in cellular activity.



2.2

Cellular Amplification



Cellular damage at the point of the initial radiation interaction usually involves
only a very small percentage of the total number of molecules in

the cell. At this stage,
there
fore, any biological consequences of radiation
-
induced changes may be relatively
insignificant. Subsequently, normal cellular metabolic processes may amplify this
damage, causing the injury to develop from the molecular to the microscopic anatomical
level,

ultimately resulting in possible gross cellular malfunction.


2.3

Gross Cellular Effects of Radiation



One of the phenomena seen most frequently in growing tissue exposed to radiation
is the cessation of cell division. This may be temporary or permanent,

depending upon the

2
-
2

magnitude of the absorbed dose of radiation. Other factors observed are chromosome
breaks, clumping of chromatin, formation of giant cells or other abnormal mitoses,
increased granularity of cytoplasm, nuclear disintegration, changes in

motility or
cytoplasmic activity, vacuolization, altered protoplasmic viscosity, and changes in
membrane permeability.



2.4

Latent Period



Following the initial radiation event, and before the first clinically detectable
effects occur, there is a time l
ag referred to as the latent period. The biological effects of
radiation are arbitrarily divided into short
-
term and long
-
term effects on the basis of the
latent period. Those effects which appear within a matter of minutes, days, or weeks are
called short
-
term effects and those which appear years, decades, and sometimes
generations later are called long
-
term effects.



2.5

The Dose
-
Effect Curve



For any biologically harmful agent, it is useful to graph the dosage administered
against the probability of ef
fect. With radiation an important question has been the nature
and shape of the resulting graph or curve. Fig. 2
-
1 below shows three dose response
relationships. Curve (1) represents a linear, nonthreshold dose
-
effect relationship in
which the curve inter
sects the abscissa at the origin. According to the nonthreshold
hypothesis, any dose, no matter how small, is considered to involve some degree of
effect. There is some evidence that the genetic effects of radiation constitute a
nonthreshold phenomenon. On
e of the underlying assumptions in the establishment of
radiation protection guides has been to take the conservative approach and consider that
any radiation absorbed will exhibit a nonthreshold effect. Under this assumption, some
degree of risk is presum
ed to be present when large populations are exposed to even very
small amounts of radiation. Curve (2) represents a nonlinear quadratic, nonthreshold
dose
-
effect relationship. This model assumes that some fraction of radiation interactions
will produce di
rect injury and that another fraction will produce subinjurious damage.
The higher the dose the more likely that several subinjurious interactins will combine to
porduce an injury and hence the superlinear increase in risk woth increasing dose. Curve
(3)

represents a “threshold dose
-
effect relationship. The point at which the curve
intersects the abscissa is the threshold dose, i.e., the dose below which there is no
immediately detectable effect.




2
-
3



Figure 2
-
1. Dose Response Relationships


2.6

Area

Ex
posed



The extent of the effect is measured by the total radiation received by the patient
and primarily depends upon the total area exposed.



Of equal importance is the nature of the organs in the body area exposed. Even
partial shielding of the radiose
nsitive blood
-
forming organs such as spleen and bone
marrow can mitigate the total effect, especially in x
-
raying children.



2.7

Variations in Cell Sensitivity



There is wide variation among different types of cells in the amount of radiation
required to

produce radiation damage. For example, cells that are rapidly dividing, or
have a potential for rapid division, are more sensitive than those which do not divide, and
cells which are nondifferentiated (i.e., primitive or nonspecialized) are more sensitive

than those which are highly specialized. The factors influencing the radiosensitivity of
cells and tissues were recognized as early as 1906 by two French scientists. Their findings
are expressed in the Law of Bergome and Tribondeau, which states:



The ra
diosensitivity of a tissue is proportional to its reproductive capacity and
inversely proportional to its degree of differentiation.


Therefore, immature cells, which are often primitive and rapidly dividing are more

2
-
4

radiosensitive than older, mature cells

which have specialized functions and have ceased
to divide.


Based upon these factors, various kinds of cells may be grouped as follows, in order of
diminishing sensitivity:


1.

Lymphocytes


2.

Erytbroblasts, granulocytes


3.

Myeloblasts


4.

Epithelial ce
lls


a.

Basel cells of the testis


b.

Basel cells of intestinal crypts


c.

Basel cells of the ovary


d.

Basel cells of the skin


e.

Basel cells of secretory glands


f.

Alveolar cells of the lungs and bile ducts


5
.

Endothelial cells


6.

Connective tissue c
ells


7.

Tubular cells of the kidney


8.

Bone cells


9.

Nerve cells


10.

Brain cells


11.

Muscle cells


2.8

Short
-
Term Effects


2.8.1

Patients



In 1994 the Food and Drug Administration (FDA) Center for Devices and
Radiological Health (CDRH) reported on re
ports of occasional, but sometimes severe,

2
-
5

radiation
-
induced skin injuries to patients resulting from prolonged , fluoroscopically
-
guided , invasive procedures. They cautioned physicians performing such procedures to
be aware that there is a potential for

serious , radiation
-
induced skin injury caused by long
periods of fluoroscopy during such procedures. They further stated that it was important
that the onset of such injuries was usually delayed and the physician should be aware that
they would not be a
ble to discern any damage by observing the patient immediately after
the treatment.



The absorbed dose in the skin required to cause skin injury depends on a number of
factors and is presented in the table below.


Radiation
-
Induced Skin Injuries



Hours o
f Fluoroscopic “On Time” to Reach Threshold
+

at:

Effect

Typical Threshold
Absorbed Dose
(Gy)
*

Usual Fluoro. Dose
Rate of 0.02 Gy/min

Time to Onset of
Effect
++

Early transient eryth.

2

1.7

hours

Temporary epilat.

3

2.5

3 wk

Main erythema

6

5.0

10 da

Pe
rmanent epilat.

7

5.8

3 wk

Dry desquamation

10

8.3

4 wk

Invasive fibrosis

10

8.3


Dermal atrophy

11

9.2

>14 wk

Telangiectasis

12

10.0

>52 wk

Moist desquama.

15

12.5

4 wk

Late erythema

15

12.5

6
-
10 wk

Dermal necrosis

18

15.0

>10 wk

Secondary ulcerat
.

20

16.7

>6 wk





The unit for absorbed dose is the gray (Gy) in the International System of units. One Gy is
equivalent to 100 rad in the traditional system of radiation units.

+

Time required to deliver the typical threshold dose at the specified do
se rate.

++

Time after single irradiation to observation of effect.


(Table adapted from Ref. 6)


2.8.2

Occupational Worker




There is a very small probability of a person’s receiving an injurious acute dose
from routine x
-
ray examinations. The dose range in
diagnostic radiographic examinations
usually varies from a few mrad to a few rad; however, in

fluoroscopic examinations, the
exposure rates seldom exceed 5

R per minute (as measured at the panel or table top), and
the entire examination seldom delivers mor
e than 30 rads. A special precaution should be
noted on radiography of the fetus: absorbed doses of less than 50 rads could result in a
spontaneous abortion.



Most of the data pertaining to the acute radiation syndrome come from animal
experimentation, bu
t there is human data which confirms the extrapolation of the animal

2
-
6

data to human populations.


The specific effects of an acute dose are dependent upon the radiation dose rate and
quality. In most mammals, there are categories of radiation death:


1.

At
very high doses, exceeding several thousand rads, death will occur with in
hours after exposure and is apparently due to the breakdown of the neurological and
cardiovascular systems; this is known as the central nervous system syndrome.


2.

At lower doses,

usually above 600 rads, death occurs within 15 to 30 days and is
associated with destruction of the gastrointestinal system.


3.

At even lower doses, but greater than 100 rads, both may occur due to radiation
effects on blood
-
forming organs. This is known

as the hemopoietic syndrome.



In general, at 50 rads or less, ordinary laboratory or clinical methods will show no
indications of permanent injury.



Considering the extent of individual variation, it is difficult to assign a precise
dose range to each o
f the syndromes discussed above. At 100 rads irradiation, most
individuals will show no symptoms, although a small percentage may show mild blood
changes. At 200 rads, most persons show definite signs of injury and

some may even die.
Approximately 600 rads

marks the threshold of the gastrointestinal form of the acute
radiation syndrome, with a very poor prognosis for all individuals involved; a fatal
outcome may well be certain at 800 to 1,000 rads whole
-
body acute irradiation. It should
be emphasized that
these estimates of injury are based on doses delivered to the whole
body in a single brief irradiation.


2.9

Long
-
Term

Effects



Long term effects of radiation are those which may manifest themselves years
after the original exposure. The latent period, th
en, is much longer than that associated
with the acute radiation syndrome. Delayed radiation effects may result from previous
acute, high
-
dose exposures or from chronic low
-
level exposures over a period of years.
From the standpoint of public health signif
icance, the possibility of long
-
term effects on
many people receiving low chronic exposures is cause for greater concern than the short
-
term effects of a few individuals receiving a high dose. This is because of possible
deleterious genetic effects.



Ther
e is no unique disease associated with the long
-
term effects of radiation; these
effects manifest themselves in human populations as a statistical increase in the incidence
of certain diseases or pathology.



Many epidemiological investigations on irradiat
ed human beings have provided
convincing evidence that ionizing radiation doses indeed result in an increased risk of

2
-
7

certain diseases long after the initial irradiation. This evidence supplements and
corroborates that gained from past and present animal e
xperimentation which
demonstrates these same effects. Among the long
-
term effects thus far observed, are

g
enetic mutations, which may be expressed many generations after the original radiation
damage, and somatic damage, which may result in increased incid
ence of cancer,
embryological effects, cataracts, and life
-
span shortening.



2.10

Carcinogenic Effects



There is human evidence that radiation may contribute to the induction of various
kinds of neoplastic disease. This evidence includes:


1.

Radium dial

painters, who ingested significant amounts of radioactive radium,
have subsequently shown an increased incidence of bone malignancies.


2.

Early radiologists and dentists have shown a significant increase in skin
malignancies and leukemias as compared to
physicians who did not use
radiation.


3.

Uranium miners have shown an increased incidence of lung cancer.


4.

The Japanese survivors of Hiroshima and Nagasaki have an increased incidence
of leukemia and possibly of other neoplasms.


2.11

Embryological Eff
ects




Considering the fact that immature, undifferentiated, and rapidly dividing cells are
highly sensitive to radiation, it is not surprising that embryonic and fetal tissues are
readily damaged by even relatively low doses of radiation. It has been sho
wn in animal
experiments that deleterious effects may be produced with doses of only ten rads
delivered to the embryo. There is no reason to believe that the human embryo is not
equally susceptible to radiation.



The specific type of fetal radiation damag
e is related to the dose and to the stage of
pregnancy during which irradiation takes place. In terms of embryonic death, the earliest
stages of pregnancy perhaps a few weeks in human beings, are most radiosensitive. From
the standpoint of practical radiat
ion protection, this early sensitivity is of great
significance, because pregnancy may well be unsuspected. Embryonic death because of
irradiation is less likely during the period of organogenesis, the second through the sixth
weeks of human gestation, tha
n in the extremely early stage, but the production of
morphological defects in the newborn is a major consideration.



During later stages of pregnancy, embryonic tissue is relatively resistant to damage
by radiation. However, functional damage, particular
ly those involving the central
nervous system, may result from such late exposure. They usually involve subtle

2
-
8

alterations in such phenomena as learning patterns and

development and may have a
considerable latent period before they manifest themselves.




2.12

Cateractogenic Effects



The fibers which comprise the lens of the eye are specialized to transmit light.
Damage to these, and particularly to the developing immature cells which give rise to
them, can result in cataracts. Radiation in sufficiently hi
gh doses can induce the formation
of cataracts; the required dose for humans is probably on the order of several hundred
rads for x
-
rays in the diagnostic energy range for a single acute irradiation.



2.13

Life
-
Span Shortening



In a number of animal expe
riments, radiation has demonstrated a life
-
span
shortening effect. The mechanisms involved in radiation life
-
span shortening are
uncertain; however, irradiated animals appear to die from the same disease as the non
-
irradiated controls but at an earlier age
. How much of the total effect is due to premature
aging and how much to an increased incidence of radiation
-
induced damage is still
unresolved.


2.14

Genetic Effect



The precursor cells of mature gametes or the mature gametes themselves are
susceptible t
o nuclear damage (genetic mutations) form external influences such as
radiation. When this occurs in those gametes which subsequently are utilized in
conception, the altered genetic information is reproduced and passed on to all of the cells
of the offspri
ng.



Most geneticists agree that the greatest preponderance of genetic mutations are
harmful. By virtue of their damaging effects, they can be gradually eliminated from the
population through natural selection. The more severe the defect produced by a giv
en
mutation, the more rapidly it will be eliminated, and vice versa; mildly damaging
mutations may require a great many generations before they disappear.



As a balance to this natural elimination of harmful mutations, fresh ones are
constantly occurring.

For man, it has been estimated that background radiation probably
produces less than ten percent of these naturally occurring mutations. Man
-
made
radiation, of course, if delivered to the gonads, can also produce mutations over and
above those which occu
r spontaneously.



Animal experimentation remains our chief source of information concerning
genetic effects of radiation. As a result of extensive experimentation, certain
generalizations may be made. Among those of health significance are:


2
-
9


1.

There is n
o indication of a threshold dose for the genetic effects of radiation,
i.e., no dose below which genetic damage does not occur.


2.

The degree of mutational damage which results from radiation exposure seems
to be dose
-
rate dependent; i.e., a given dose is

less effective in producing
damage if it is protracted or fractionated over a long period of time, due to cell
and tissue repair.




Radiation and other mutagenic factors have always been present on earth. It is reasonable
to expect that all mutations hav
e been expressed in

the past so that man
-
made radiation
would only add to the natural incidence of previously expressed mutations rather than
create new ones. Since, in general, mutations tend to be deleterious, it is important to
keep their incidence as l
ow as possible. Therefore, the goal is clear; we should keep the
radiation exposure of the gonads to a minimum.







3
-
1

UNIT 3:
RADIATION
EXPOSURE STANDARDS


3.1

Radiation Quantities and Units



The production of a radiographic image is dependent upon the ab
sorption of radiation by
the patient, or more precisely, the selective absorption of radiation throughout the irradiated
tissue of the patient. It is the differential absorption of different structures that provides the
desired information. Unfortunately,
the absorption of energy from the X
-
ray beam can have
deleterious effects on tissue. It is this dilemma that confronts every licensed operator: X
-
rays
must be absorbed by the patient to produce an image, but the absorption of X
-
rays can produce
undesirable

effects, both somatic and genetic. The logical response to this dilemma is to try to
maximize radiological information while minimizing radiation exposure to the patient.



A number of acute and long
-
term effects on humans have been related to the physica
l
energy absorbed from various types of ionizing radiation. However, the relative effectiveness of
each type of radiation per unit energy absorbed in tissue has been found to vary not only with the
type of radiation and its quantum energy but also with the

rate which the energy is delivered, type
of tissue irradiated, the age of the patient, the biological effect under consideration, and other
epidemiological variables.



In order for radiation dose measurements to be meaningful, they must be related to som
e
biological effect of interest. We know that absorption of X
-
rays produces free electrical charges
in air by

ionizing air molecules. Such ionization requires the absorption of energy from the
radiation. We now know that many deleterious biological effects

can be related to absorbed
energy in tissue.



The measurement of X
-
rays is important in any quantitative investigation of their
properties. It is essential to distinguish, however, between a “quantity”, defined as the description
of a physical concept or

principle, and a “unit”, which is the measure of magnitude of the
quantity.



For the purposes of radiation hazard evaluation, various units, of radiation exposure and
dose have been introduced to account for the several methods of measuring and assessing

the
effects of different types of radiation.



In 1975
,
the scientific community within the United States agreed to adopt the
“International System of Units or SI” for measuring and assessing the effects of the different
types of radiation. Four of the mo
st important quantities and corresponding conventional and
approximate equal SI units are: Exposure
-

Roentgen (coulounb per kilogram); Absorbed Dose
-

Rad (Gray); Dose Equivalent
-

Rem (Sievert) and Activity
-

Curie (Becquerel). The conventional
units a
re not directly equivalent to the SI system, therefore, arithmetic conversions are necessary
in order to go from one system to the other.



3
-
2

3.1.1

The Roentgen:




The Roentgen (abbreviated R, and its corresponding one
-
thousandth, the milliroentgen,
mR) is t
he unit of exposure and applies only to the interaction between X and gamma radiation
and air. It is applicable only for photon energies not exceeding three million electron volts
(MeV). Although historically the oldest and most widely used unit of X
-
ray m
easurement, it has
no direct biological context and is used in the calibration of radiation
-
producing machines as a
means of specifying their output intensity. Usually, if the exposure is known in roentgens, the
dose in rads can be computed if the X
-
ray be
am size, and other necessary factors are known. The
QUANTITY of X
-
rays in a beam is frequently expressed as EXPOSURE RATE, i.e., Roentgens
per minute (R/m) or milliroentgens per second (mR/s).



Roentgen is a “unit of exposure of X or gamma radiation. One
roentgen is the exposure
corresponding to ionization in air of one electrostatic unit of charge either sign in 0.001293 gram
of air”.



3.1.2

The Coulomb per Kilogram:



The Coulomb per Kilogram (abbreviated C kg
-
1
) is the quantity of x or gamma radiation
that imparts a coulomb of energy to a kilogram of air producing ions (of either sign).


1R = 2.5 x 10
-
4

C kg
-
1


3876R = 1 C kg
-
1


3.1.3

The Rad:



The Rad, an acronym for Radiation Absorbed Dose, is the special unit of absorbed dose.
The quantity signific
ant in biological and medical work is not the amount of radiation passing
through a point in air; rather, it is the amount of energy absorbed by the substance at the
particular point, i.e., the absorbed dose. An absorbed dose of one rad corresponds to the
absorption of 100 ergs of energy per gram of tissue or other material and is of primary
importance in radiation dosimetry. (There is no abbreviation for rad or rem; initially a lower
-
case
r was used for roentgen; an upper
-
case R is now used for roentgen,

and hence there is no symbol

for rad or rem).


3.1.4

The Gray:



The Gray (abbreviated Gy) is an absorbed radiation dose of one joule per kilogram in
issue.

1 rad = 10
-
2

Gy


100 rad = 1 Gy



3
-
3

3.1.5

The Rem:



Dose Equivalent: This unit (the rem) was devised to
allow for the fact that the same
absorbed dose in rads delivered by different kinds of radiation does not produce the same degree
of biologic effect; some radiations are biologically more effective than others. For protection
purposes, where a mixture of r
adiations may have to be considered, allowance is made for this by
the quality factor which relates the effect of other radiations to that of gamma rays from cobalt
-
60. Some quality factors are listed below:


Quality Factor

X
-
rays, gamma rays, and electro
ns (including beta rays)

1

Fast neutrons and protons of energies up to ten MeV

10

Alpha particles

20



The dose equivalent in rem is the absorbed dose in rads multiplied by the appropriate
“quality factor.” Permissible dose equivalent of radiation is speci
fied in rem. The determination
of dose equivalent is especially important when considering doses to critical organs.
Occupational dose equivalent limits (maximum permissible dose, MPD) are all stated in terms of
rem.



It is important to make a distinction

between dose measured in rads, exposure measured in
roentgens, and dose equivalent measured in rem; however, their biological impact is evaluated in
rem.


“Rem” is an acronym for Roentgen Equivalent Man.


3.1.6

The Sievert:



The Sievert (abbreviated Sv)

is the dose equivalent and is equal to the absorbed dose in
grays multiplied by the appropriate “quality factor” and other factors.


1 rem = 10
-
2

Sv


100 rem = 1 Sv


3.1.7 The Curie (Ci):



The curie is used to specify the activity of a radionuclide, i/e
., the rate at which its atoms
disintegrate. One curie equals 3.70 x 10
10

disintegrations per second.



3
-
4

3.1.8 The Becquerel:



The Becquerel (abbreviated Bq) is that quantity of radioactive material in which one atom
is transformed per second.


1 Ci = 3.7
x 10
10

Bq



2.7x10
-
11
Ci = l Bq



3.2

Maximum Permissible Exposure
-
MPD (Dose Equivalent)



In total, natural radiation in the United States results in an estimated average annual dose
equivalent of about 300 mrem. It is unlikely to be less than 100 mrem fo
r any individual and unlikely
to be more than 400 mrem for any significant number of people.



The essential aim of radiation safety is to prevent injury from ionizing radiation. Three types
of dose equivalent limits are:


1.

Occupational dose equivalent l
imits for persons over 18 years of age.


2.

Occupational dose equivalent limits for persons under 18 years of age.


3.

Dose equivalent limits for general population.



However, the New York State Department of Health, Bureau of Environmental Radiation
Prot
ection in its Regulation on Ionizing Radiation, Chapter 1


Part 16, dated April 2001
,
also refers
to prenatal radiation exposure.



Before discussing the regulatory provisions regarding permissible dose equivalent limits, one
should review definitions of

occupational dose. Occupational dose is defined as the dose received by
any individual in:


a.

A controlled area (X
-
ray room), or
1/


b.

The course of employment, education, training, or other activities which involve exposure
to radiation.


Exception: Rad
iation exposure received for the operator’s own personal medical or dental diagnosis
or medical therapy is not considered to be occupational exposure. If an operator is a patient, he/she
must remove the personnel monitoring device before being X
-
rayed.



3
.3

Occupational and General Population Dose Equivalent Limits




1/

Controlled area means an X
-
ray room or any other area where radiation safety rules are enforced.



3
-
5



The basic provisions regarding the dose equivalent limits for occupationally exposed
persons and the general public are given in the table below:


REM PER CALENDAR QUARTER
2/



Occupati
onal Dose


Body Area

Over 18

Under 18

General Public


Whole body

1.25

0.125


0.100 per year


Skin and any extremity

12.5


1.25



Eyes

3.75

0.375



3.4


Retrospective Annual Occupational Dose Equivalent



An occupationally exposed individual over 18 y
ears of age may receive, on the average,
a maximum whole
-
body dose equivalent of 5 rem per year.



3.5

Who

Must Be Monitored



The question of who must be monitored and under what conditions persons must be
monitored confronts every certified supervisor
(registered user) whose employees run a risk of
exposure to radiation. As is often the case with regulatory provisions, there are many implied
provisions. To a very great extent, this is true with personnel monitoring requirements. We feel
that the questio
n you should ask yourself is: Would you, as a certified (registered user), want the
responsibility of risking any person’s safety by not monitoring that person?


The clearly stated personnel monitoring requirements are:


Each registered user must supply a
ppropriate personnel monitoring equipment to, and shall
require the use of such equipment by, any person who is likely to receive an accumulated dose
equivalent in excess of 10 percent of the applicable annual radiation exposure standard specified
by regul
ation, as listed in the table below:



REM FOR CALENDAR YEAR


Body Area



Over 18

Under 18


Whole body

5

0.5




2/
”Calendar quarter” means not fewer than 12
consecutive weeks and not more than 14 consecutive weeks. Calendar
quarters shall be so arranged that no day in any year is omitted from inclusion within a calendar. No user shall
change the method omitted observed by him of determining calendar quarters

except at the beginning of a calendar
year.



3
-
6

Skin or extemities


50

5

Eyes


15

1.5




There are two additional personnel monitoring requirements: (1) for persons who enter
high radiation area
s, and (2) for persons who operate mobile X
-
ray equipment.
3/



In addition, there are two broad provisions which deserve emphasis: (1) each user
(certified supervisor) must take all precautions necessary to provide reasonably adequate
protection to the lif
e, health, and safety of all individuals subject to exposure to radiation, and (2)
each certified supervisor is responsible for radiation safety in his X
-
ray department.



3.6

Occupationally Exposed Women of Procreative Age



A special situation arises wi
th occupationally exposed declared pregnant women.
NYSDOH Part 16 states that special precautions shall be taken to limit exposure to declared
pregnant women, especially if they could be pregnant. Exposure to the abdomen of such workers
to X
-
rays would inv
olve exposure to the embryo or fetus.


You, as a certified supervisor, are responsible for instructing the employee of the following.


1.

That the NYSDOH states that during the entire gestation period, the maximum
permissible dose equivalent to the fetus f
rom occupational exposure of the declared
expectant mother does not exceed 0.5 rem, and that the working conditions be adjusted
so as to avoid a monthly total effective dose equivalent of more than 0.05 rem to the
embryo/fetus and


2.

Provide the employee

with reasons for the recommendation.



It is strongly suggested that the instruction be given both orally and in writing. Also, each
individual should be given an opportunity to ask questions, and each individual should be asked
to acknowledge in writing
that the instruction has been received.



Some recent studies have shown that there is an increased risk of leukemia and other
cancers in children if the expectant mother was exposed to a significant amount of radiation. The
NYSDOH wants women employees to

be aware of any possible risk so that the women can take
steps they think appropriate to protect their offspring.


The following facts should be given to the woman employee:


1.

The first three months of pregnancy are the most important, because the embry
o or



3/


Radiation area” means any area accessible to individuals, where a major portion of the body could
receive a dose exceeding 5

millirem in any 1 hour or 100 millirem in any 5

consecutive days.


“High radiatio
n” area means any area, accessible to individuals, in which there exists radiation at such
levels that an individual could receive in any 1 hour dose to the whole body in excess of 100 millirem.



3
-
7

fetus is very sensitive to radiation.


2.

In most cases of occupational exposure, the actual dose received by the embryo or fetus
is less than the dose received by the mother, because some of the dose is absorbed by
the mother’s body.


3.

At the prese
nt occupational dose equivalent limits, the risk to the unborn baby is
considered to be small, but experts disagree on the exact amount of risk.


4.

There is no need for women to be concerned about sterility or loss of ability to bear
children.


5.

The NCRP reco
mmendation of 0.5rem dose equivalent limit applies to the full nine
months of pregnancy.











4
-
1

UNIT 4: RADIATION PROTECTION


4.1

Terminology


DOSE
-
RESPONSE CURVES: The linear dose
-
response curve is characterized by an effect
caused directly by a specif
ic dose and no threshold. The sigmoid dose
-
response curve is
characterized by a representation of nonstochastic effects or a non
-
dose effect relationship.


BACKGROUND RADIATION: That radiation due to cosmic rays natural or environmental
radioactivity, whic
h is always present.


DOSE RATE: An important aspect of irradiation is the dose rate, which is the dose delivered per
unit time. Dose rate is expressed in rem per hour, the absorbed dose in rad/h, and exposure in
R/h.


HALF
-
VALUE LAYER (HVL): The QUALITY (
average penetrating ability) of an X
-
ray beam
is usually specified terms of half
-
value layer. The HVL is defined as the thickness or layer of a
specified material which attenuates the X
-
ray beam to such an extent that the exposure rate is
reduced to one ha
lf; e.g., 1 HVL reduces the exposure level by 1/2, 2 HVLs reduce the exposure
level by 1/2 x 1/2 or 1/4, etc.



4.2

Conduct of the Examination


4.2.1

Introduction



The manner in

which that examination is

conducted

will determine

the patient’s dose, one
wa
y to

minimize the patients dose, dose to the operator
,
and at the same time improve image
quality is to restrict the size of the exposure field by coning, collimation, and general patient
shielding
--
particularly for such areas as the fetus, gonads, lens of

the eye, and active blood
-
forming organs.



Only that part of the X
-
ray beam which has gone though the patient and reaches the image
receptor (called remnant X
rays) can produce a radiograph with proper density, contrast, and
detail and thus provide diagn
ostic information. Failure to limit the X
-
ray beam only to the area of
clinical interest represents one of the most frequent causes of unnecessary patient radiation.



Most modern X
-
ray machines are equipped with adjustable collimating devices that can be
used to restrict the size and shape of the X
-
ray beam. Most collimating devices are equipped also
with a light localizer, which provides a visual indication of size and location of the X
-
ray beam at
any distance.



All these devices have an important funct
ion to perform in addition to limiting patient
dose
--

they improve the contrast and detail of the radiographic image by reducing the amount of
scattered radiation reaching the film and the operator. Therefore, it is vital that the operator
properly employ

these devices at all times.


4
-
2


4.3

Fluoroscopic Examinations



4.3.1



Introduction



Since Thomas A. Edison invented the fluoro
scope in 1896, it has been a valuable tool in
the practice of medicine. The primary function of the fluoroscope is to perform dyna
mic studies;
that is, the fluoroscope is used to visualize the motion of internal structures and fluids.

Dur
ing
fluoroscopy, the radiologist views a contin
uous image of the motion of internal structures while
the x
-
ray tube is energized. X rays produced
in the X
-
ray tube spread out (fan out) from the
source and pass through the patient. The patient’s anatomy filters the X rays. Some of the X rays
completely penetrate the patient. When the X rays complete their passage through the patient, an
image in the
form of an X
-
ray field of spatially varying intensities is produced. This is the X
-
ray
image. If something is observed that the radiologist would like to preserve for later study, a
radiograph can be made with little interruption of the fluoroscopic examin
ation. Such a
radiograph is known as a spot film.




Fluoroscopic procedures may be performed by certified physicians properly trained in
fluoroscopic techniques. The regulation specifies that only persons who have been adequately
instructed in safe operat
ing procedures and who are competent in the safe use of the equipment
may operate it.



Fluoroscopy is actually a rather routine type of x
-
ray examination except for its application
in the visualization of vessels, called angiog
raphy. The two main areas o
f
angiography are neuroradiology and vascular radiology, and, as with all fluoroscopic procedures,
spot film radiographs are obtained also. The recent intro
duction of computer technology into
fluoroscopy and radiography is placing increasing demands on t
he training and performance of
radiologic technologists.



Furthermore, fluoroscopic examinations should be performed only after careful
consideration, because fluoroscopic examinations could expose the patient to much larger
quantities of radiation than r
adiographic examinations. For example, an upper GI tract
fluoroscopic study utilizing 120 seconds actual exposure time could deliver at the patient as
much as 10 to 15 Roentgens, as compared to an AP abdominal film where the exposure range,
according to th
e Nationwide Evaluation of X
-
Ray Trends (NEXT) data, is 100 to 750
milliroentgens at the patient.


Two facts should be kept in mind when operating a fluoroscope:


1.

Operator exposure to scattered radiation is directly proportional to patient exposure.


2.

Ima
ge brightness is directly proportional to the radiation exposure rate at the output
phosphor.



4
-
3


The layout of a modern fluoroscopic system is shown in Fig. 4
-
2. The x
-
ray tube is usually
hidden under the patient couch. Over the patient couch is the image

intensifier and other image
detection devices. The image intensifier (II) converts an radiologic image into a light image that
is displayed on a television monitor. Modern fluoroscopic equipment allows the radiologist to
select an image brightness level
manually or maintained automatically by the machine varying
the kVp or the mA, or sometimes both. Such a feature of the fluoroscope is called automatic
rightness control (ABC), automatic brightne
ss stabilization (ABS), automatic exposure control
(AEC), o
r automatic gain control (AGC). Other fluoroscopes have the x
-
ray tube over the patient
couch and the image receptors under the patient couch. Some fluoro
scopes are operated remotely

from outside the x
-
ray room.

Figure 4
-
2. Fluoroscopic X
-
ray System



The radiation exposure rate at the output phosphor will increase with increased X
-
ray tube
current in milliamperes. This will produce a brighter image on the screen (image receptor), but
will also increase patient exposure, and hence operator exposure. Ra
diation exposure rate at the
output phosphor is almost independent of the X
-
ray beam size. Consequently, the image will not
be brighter with a larger X
-
ray beam size; however, the total volume of the patient that is
exposed to radiation will increase, and
with it, the amount of radiation scatter toward the
operator. On the other hand, image quality is improved as the size of the X
-
ray beam is reduced,
because there is a reduction in the amount of scattered radiation reaching the output phosphor.




It shoul
d be possible to reduce patient and operator exposure and still obtain a satisfactory
fluoroscopic image for viewing purposes by fulfilling the following conditions:


1.

Use the lowest milliamperage and optimum peak kilovoltage technique possible.


2.

Rest
rict the X
-
ray beam to the smallest size practicable.


3.

Keep the X
-
ray tube to patient distance to a maximum.


4.

Restrict X
-
ray beam ‘ON” time to a minimum.



4
-
4

Factors which influence the exposure rate at the table top, and hence influence the dose to bot
h
the patient and the operator, are:


Milliamperage or tube current.


Collimation.


Patient Shielding


Exposure time.


X
-
ray tube to patient distance.


Light in the fluoro room.


Grid use.


4.3.2

Milliamperage



For a radiographic examination, the x
-
ray t
ube current is measured in hundreds of mA
and the X
-
ray output is proportional to the mA used. If the mA setting is reduced from 5mA to
3mA, the exposure rate is correspondingly reduced to 40 percent of the initial exposure rate.
During fluoroscopy, the
x
-
ray tube is operated at less than 5

mA. When image intensification was
first introduced, it was anticipated that tube current could be reduced by at least a factor of ten
and that as a result patient dose would be reduced by a factor of ten. For a variet
y of reasons this
tube
-
current reduction has not materialized. During image
-
intensified fluoroscopy, tube cur
rents
of 2 to 4 mA are normal. Consequently, the patient dose during fluoroscopy remains relatively
high, considerably higher than doses resulting

from radiographic examinations.


There are certain regulatory provisions which must be observed:


1.

For routine fluoroscopy, the exposure rate measured at the panel or table top shall be as
low as practicable and may not exceed 10 roentgen per minute.


2
.

Devices which indicate the X
-
ray tube potential and current must be provided, and should
be located in such a manner that the operator may monitor the tube potential and current during
fluoroscopy.


3.

Periodic measurements of the exposure rate at the ta
ble top must be made.


4.3.3

Collimation



The restriction of the useful x
-
ray beam to reduce patient dose and improve image
contrast is known as
collimation
. Collimators take many different forms. Blade
-
type,
diaphragms, adjustable light
-
localizing co
llimators, and cones are the most frequently used

4
-
5

colllimating devices. They also reduce scatter radiation and thus enhances image contrast. An
X
-
ray beam should never be larger than the film size used; e.g., a 24 inch diameter circle exposes
a body area

of about 18 inches by 24 inches, or 432 square inches; a 36 inch diameter circle of
radiation exposes a body area of approximately 18 inches by 36 inches, or by 648 square inches.
A well
-
collimated X
-
ray beam covering 14 inches by 17 inches exposes only 2
38 square inches.



As may be readily ascertained, reduction in X
-
ray exposure is considerable where
rectangular collimation is used. Even more striking reduction of exposure could occur during an
AP lumbar spine X
-
ray examination if a well
-
collimated X
-
ra
y beam is used instead of a circular
cone of 24 inch diameter. The 24 inch diameter circular cone covers approximately 432 square
inches of body area, whereas a well
-
collimated X
-
ray beam of 7 inches by 14 inches covers only
98 square inches. Exposure fiel
d reduction and the reduction of skin exposure is considerable:







%
77
432
77
432



Conversely

%
340
98
98
432



excess skin exposure

if a 24 inch circular
cone is used.


Consequently, the importance of a rectangular collimating device and

its correct use cannot be
overemphasized. As a certified supervisor, it is your responsibility to enforce the regulatory
provision which requires the radiographic field (X
-
ray beam) restriction to the area of clinical
interest only. During fluoroscopy,
you should restrict the X
-
ray beam to the smallest size
practicable for the examination at hand. Doubling the exposure area also doubles patient
exposure.


4.3.4

Gonad Shielding


Suitable protective devices, as stipulated in the U.S. Department of Health,
Education, and
Welfare publication, entitled “Gonad Shielding in Diagnostic Radiology,” must be provided to
shield gonads in potentially procerative patients when gonads cannot be excluded from the X
-
ray
beam and the shielding of the gonads does not interf
ere with the diagnosis. The gonadal shielding
may not be less than 0.5mm lead equivalent.



The use of 0.5mm lead equivalent gonad shielding reduces gonad dose by approximately
97 percent, e.g., for a primary X
-
ray beam of l00kVp and 3mm aluminum filter, t
he transmission
throughout the shield is 3 percent, assuming that the shielding material encloses the testes. The
testes under a lead sheet gonad shield can receive internally scattered radiation up to about five
percent of the incident primary X
-
ray beam.

Therefore, total gonad dose reduction for a 0.5mm

sheet of lead (or shadow field) is 97%
-

5%

= 92%.



The ovaries in female patients are situated in the abdomen at varying depths so that
shielding would more frequently interfere with diagnosis. However,
whenever possible gonad
shielding appropriate for females should also be utilized.


4.3.5

Exposure Time



4
-
6


You should restrict the X
-
ray beam “ON” time to a minimum. Doubling the exposure
time also doubles the exposure. Patient and operator exposure will in
crease with prolonged beam
“ON” time. Usually, the X
-
ray beam need not be on continuously, and fluoroscopy can be
accomplished with a series of short spurts of x
-
radiation. A cumulative manual
-
reset timer
activated by the exposure switch that produces an

audible signal or temporarily interrupts the X
-
ray beam when the fluoroscopy time has exceeded a predetermined time limit, not to exceed five
minutes, must be provided. This device is designed to make sure that the fluoroscopist is aware
of the relative b
eam “ON’ time during each procedure, and is primarily for the patient protection.


4.3.6

Magnification and X
-
ray Tube
-
to
-
Patient or Image Intensifier
-
to
-
Patient Distance



All images on the radiograph are larger than the objects they represent, a conditio
n called
magnification. (Distortion, however, is unequal magnification of different portions of the same
object.) Therefore, use of an optimum field size is important for the efficiency and effectiveness
of a procedure as well as minimizing the entrance sk
in exposure to the patient. Standard field
sizes for some modes of operation are 4, 6, 9, or 12 inches. Some may only have one size and
typically it is 9 inches in diameter. The smaller the field size, the more magnification appears on
the TV monitor.
The entrance skin exposure increases if the image is magnified and the effect
may be different for different systems. A good rule of thumb is to use the smallest magnification
consistent with the procedure with close collimation.



The x
-
ray tube
-
to
-
patie
nt (tabletop) distance must be not less than 15 inches on stationary
fluoroscopic units and not less than 12 inches on mobile fluoroscopic units. Increasing the
distance between the fluoroscopic tube and the patient results in reduced patient dose because

of
the corresponding decrease in the difference between the entrance and exit dose to the patient.


4.3.7

Excessive Light


Provisions must be made to eliminate extraneous light that interferes with the fluoroscopic
examination.


4.3.8

Grids



The grid is
an extremely effective device for reducing the level of scatter radiation, it
selectively shields the image intensifier from scattered X
-
ray. It can be either manually removed
as on many C
-
arms or automatically removed from the beam as on many GI units.
When
manually removing it care should be taken to so as not to nick or dent the grid and ruin the
effectiveness of the grid. It is a carefully fabricated series of sections of radiopaque material
alternating with sections of radiolucent material. The gri
d is designed to transmit only those x
-
rays whose direction is on a straight line from the X
-
ray tube to the image receptor. Because it
reduces the amount of scatter reaching the II it can improve the quality of the image, however,
the patient’s radiation

dose can increase. It could increase the patient dose by a factor of 2 or
more. This may be alright if image quality is a necessity. A grid may not be required for
fluoroscopic procedures involving either pediatric patients or small adults particularly

if the II
can’t be brought closer than 25 cm to the patient.


4
-
7


4.3.9

Bucky Slot Cover



During fluoroscopy, the Bucky tray is moved to the end of the examination table, leaving
an opening in the side of the table approximately two inches wide at the gonada
l level. This
opening must be automatically covered with at least 0.25mm lead equivalent material.


4.3.10

Protective Curtain



A protective curtain (overlapping protective drapes) or hinged or sliding panel of at least
0.25mm lead equivalent should be pos
itioned between the patient and the fluoroscopist or others
who are required to remain in the room during exposure.


4.3.11

Operator Protection During Fluoroscopic Examinations


The following guidelines are provided:


1.

Protective aprons of at least 0.25
mm lead equivalent (preferably 0.5mm) must be worn in
the fluoroscopy room by each person unless they stand behind a radiation barrier, except the
patient. Usually, leaded eye protection or thyroid protection are not required, except for high
dose rate te
chniques. It is most unlikely that the risk of induced cataracts should be a concern, as
the threshold dose is so large for chronic exposure. However, patient shielding of gonadal area
should be provided, if appropriate.


2.

The operator shall monitor th
e X
-
ray tube current and potential at least once each day
during the use to ascertain that they are in the normal range, and keep logs of all such daily
monitored readings.


Personnel monitoring devices shall be worn by anyone routinely performing fluorosc
opic
procedures (IF only one monitoring device is worn, it should be located at the collar of the apron
or on the outside of the thyroid shield.)


Bucky slot cover and protective curtain must be provided on conventional fluoroscopic units.


Physicians shou
ld never put their hands into the active beam.


4.3.12

C
-
Arm Fluoroscopy



In C
-
arm fluoroscopy there are no shielded drapes, no shielded table, and the
examinations are usually performed in the room. In these configurations it is very important that
the
operator pay attention to radiation management practices specific to this type of unit.



With the C
-
arm oriented vertically, the X
-
ray tube should be located beneath the patient
and the image intensifier above. In a lateral or oblique orientation, the X
-
ray tube should be
positioned opposite the area where the operator and other personnel are located. The operator

4
-
8

and the image intensifier should be located on the same side of the patient. The orientation of the
X
-
ray system is essential because it take
s advantage of the patient as a protective shield and it
reduces the amount of radiation scattered from the patient. If additional personnel are required in
the room during the procedure, they should be positioned on the X
-
ray tube side and should stand
b
ehind a mobile shield. Any person in the procedure room and who is not behind a shielded
barrier must wear a lead apron and may need to have a radiation monitoring badge. Persons not
directly involved in the procedure stand behind a protective barrier or

step back from the X
-
ray
field at least 6 feet or leave the room.


Review of the components and their effect during a Fluoroscopic Exam


Component element

Radiation and Quality Effects


Image Quality

Dose to the Patient

Scatter Dose to the Staff

Size of

Patient

Decrease

Increase

Increase

Increase Tube Current
(mA)


Increase


Increase


Increase

Increase kVp

Decrease (reduced
contrast)

Decreased if mA is
appropriately decreased

Depends on the tube
current

Increase Tube
-
to
-
Patient
Distance

Increased Usua
lly
(depends on focal spot
size)


Decrease

No Significant Change


Increase Image
Intensifier
-
to
-
Patient
Distance

Decrease Usually
(depends on focal spot
size)


Increase

No Significant Change

Increase Image
Magnification


Increase

Increase Usually
(depend
s on equipment
design)

Not Much Change
Usually (depends on
equipment)

Grid Used

Increase if II is close to
adult patient)

Increase

Increase

Increase Collimator
Opening

Decrease

About the same dose,
more tissue exposed

Increase

Increase Shielding of
Room

and Staff

No Effect

No Effect

Decrease

Increase Beam On
-
Time

No Effect

Increase

Increase



4
-
9

4.4

Time
-

Distance
-

Shielding


There are three basic principles, which can be used singly or in combination, to reduce dose to x
-
radiation. These are:


1.

Time
-

keep the time of exposure as short as practicable.

2.

Distance
-

keep the distance between the source of exposure (X
-
ray tube or any scattering
medium such as a patient) and the exposed individual as large as practicable.


3.

Shielding
-

insert shielding
material between the source of radiation and the exposed
person, as applicable.


4.4.1

Time



The exposure times in radiographic work usually are predetermined. During fluoroscopic
examinations, the dose to the patient is directly related to the dose rate
and the duration (time) of
exposure. Also, the greatest operator exposure to scattered radiation is directly proportional to
patient exposure. The cumulative manual
-
reset timer has been specifically designed to make the
fluoroscopist aware of the relativ
e X
-
ray beam “ON” time during each fluoroscopic procedure.


4.4.2

Distance



The intensity of radiation varies inversely as the square of the distance. It is obvious that
the farther the person is from the X
-
ray source, the less radiation dose per unit of

time they will
receive. This is why in radiographic work the target
-
to
-
skin (usually the target
-
to
-
film) distances
are relatively long (usually 40 inches). Also, the target
-
to
-
tabletop distances in fluoroscopy
usually are as long as practicable (not less
than 15 inches for stationary and not less than 12
inches for mobile units).



Inverse square law: At points distant from a common source of x
-
radiation, the
intensities of radiation at these points vary as the square of their respective distances from th
e X
-
ray source. As one moves farther away from an X
-
ray source, the less radiation they receive,
because the X
-
ray beam diverges as it moves away from its source. The inverse square law can
be expressed as a simple mathematical relationship and as shown in

Fig. 4.3:



4
-
10


E
1

(D
2
)
2




=



or E
1

x (D
1
)
2

= E
2

x (D
2
)
2


E
2


(D
1
)
2


Where:

E
1

-

intensity at distance D
1




E
2

-

intensity at distance D
2











Figure 4.3. Inverse Square Law



It is easy to see that if the distance from an X
-
ray
source is doubled, the radiation intensity
is reduced to 1/4 of the intensity at the original distance. If the distance from the source is tripled,
the intensity is reduced to 1/9. If the distance from source is quadrupled, the intensity is 1/16, etc.



Mo
st radiation sources are point sources, the X
-
ray tube target, for example. However, the
scattered radiation generated within the patient during an X
-
ray exposure comes from an
extended area.



In radiography, the distance from the X
-
ray tube to the patien
t is generally fixed by the
type of examination, and the technologist stands behind a protective barrier. During special
procedures work or during fluoroscopy, the technologist may be required to remain in the X
-
ray
room during the exposure. In such cases,

it is wise to remember the configuration of isoexposure
curves and stand where the radiation levels would be the lowest. As a rule of thumb, the
technologist should remain as far from the examination table as practical.



During portable work, the technol
ogist should stand as far away from the patient as
practical. This is why the control switch cord must be at least six feet long.


4.4.3

Shielding



Shielding is one of the most important principles for radiation protection. Shielding refers
to the differe
nt means used to stop radiation or to prevent exposure to it. To be able to apply
shielding methods, one must have some understanding of the manner in which x
-
radiation is
attenuated (absorbed) in an absorbing medium. Energy is lost by three methods. The t
hree
methods are: the photoelectric effect (a collision between a photon of x
-
radiation and an orbital
electron of an atom where the electron is knocked out of its orbit and the photon loses all its
energy); Compton scattering (interaction of a photon of x
-
radiation with an orbital electron of the

4
-
11

absorber atom producing a recoil electron and a photon of energy which is less than that of the
incident photon); and pair production (an incident photon is annihilated in the vicinity of the
nucleus of the absor
bing atom, with subsequent production of an electron and positron pair). The
photoelectric effect is most important at low energies (up to 100k kVp), Compton scattering at
intermediate energies and pair production at high energies (above 1,022 kilovolts).



As X.rays pass through an absorber, their decrease in number is governed by the energy of
radiation, the specific medium, and the thickness of the absorber traversed. Mathematically, the
absorption may be expressed by the equation:


I = I
0

e
-
ux



where

I

-

intensity after absorption

I
0

-

incident intensity



u
-

absorption coefficient



x
-

thickness of absorber traversed



e
-

natural logarithm base = 2.72



In discussing shielding, there are a few facts to keep in mind: (1) persons outside the
shadow
cast by the shield are not protected; (2) a wall or partition is not necessarily a safe shield
for persons on the other side; and (3) radiation can bounce around corners; that is, it can be
scattered.



The third fact is so important that it merits further

clarification. Scattered radiation is
present to some extent whenever an absorbing medium is in the path of radiation. The absorber
(patient during irradiation) then acts as a new source of radiation. Frequently, room walls, the
floor, and other solid obj
ects are near enough to a source of radiation to make scatter appreciable.



Certain factors determine the quantity of scatter radiation. These are as follows: (1)
kilovoltage, (2) part thickness, and (3) field size. Scatter radiation is maximum with high
kVp
techniques, large fields, and thick parts, and unfortunately, this is what we usually deal with in
diagnostic radiology. We rarely have any control over part thickness and frequently must use
large fields. The only variable we can control is kVp, but e
ven here we have less control than we
would like since patient doses go up sharply with low kVp techniques.



The Center for Devices and Radiological Health, an agency of the Food and Drug
Administration, has the responsibility for conducting the regulato
ry program. Its authority is
limited to regulating the manufacture and repair of equipment and is exercised through its
promulgation of performance standards. Standards pertaining to ionizing radiation have been
issued for diagnostic X
-
ray systems and thei
r major components, television receivers, gas
discharge tubes, and cabinet X
-
ray systems, including X
-
ray baggage systems. It performs
surveys on the exposure of the population to medical radiation. It conducts an active educational
program on the proper u
se of X
-
rays for medical purposes and has completed teaching aids for
X
-
ray technicians, medical students, and residents in radiology. It has published
recommendations for quality assurance programs at medical radiological facilities to minimize
patient ex
posure. It has also issued standards pertaining to nonionizing radiation, such as
microwaves and laser beams.


4
-
12

BIBLIOGRAPHY


1.

Curry, Thomas S., Dowdcy, James E., Murry, Robert C.,
Christensen’s Introduction

to

the

Physics

ot
Diagnostic

Radiology
, Lea an
d Febiger, Philadelphia, PA.


2.

“SI Units in Radiation Protection and Measurements”. National Council on Radiation Protection and
Measurements Washington, D.C.: NCRP Report No. 82, 1985.


3.

“Syllabus on Diagnostic X
-
ray Radiation Protection for Certified

X
-
ray Supervisors and Operators”.
State of California, Department of Health Services, Radiologic Health Section.


4.

Bell, Roy, 1001 Questions About Radiologic Technology”. Volumes 3 & 4, University Park Press,
Baltimore, MD.


5.

Bushberg, Jerrold T., Sei
bert, Anthony J., Leidholdt, Edwin M., Jr., Boone, John M.,
The Essential
Physics of Medcial Imaging
, Williams & Wilkins, Baltimore, M.D.


6.

“FDA Public Health Advisory: Avoidance of Serioius X
-
Ray
-
Induced Skin Injuries to Patients
During Fluoroscopicall
y
-
Guided Procedures”. US Food and Drug Administration, Rockville, M.D.,
1994.